Design Note DN0007 2.4 GHz Inverted F Antenna

By Audun Andersen Keywords • CC2400 • CC2420 • CC2430 • CC2431 • CC2500 • CC2510 • CC2511

• CC2550 • CC2520 • CC2480 • PCB Antenna • 2.4 GHz • Inverted F Antenna

1 Introduction

This document describes a PCB antenna design that can be used with all 2.4 GHz transceivers and transmitters from Texas Instruments. Maximum gain is measured

to be +3.3 dB and overall size requirements for this antenna are 25.7 x 7.5 mm. Thus, this is a compact, low cost and high performance antenna.

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Design Note DN0007 Table of Contents KEYWORDS.............................................................................................................................. 1 1 INTRODUCTION............................................................................................................. 1 2 ABBREVIATIONS........................................................................................................... 2 3 DESCRIPTION OF THE INVERTED F ANTENNA DESIGN ......................................... 3

3.1 IMPLEMENTATION OF THE INVERTED F ANTENNA.......................................................... 3 4 RESULTS........................................................................................................................ 4

4.1 RADIATION PATTERN.................................................................................................. 4 4.2 REFLECTION ............................................................................................................ 11 4.3 BANDWIDTH............................................................................................................. 11

5 CONCLUSION .............................................................................................................. 12 6 REFERENCES.............................................................................................................. 13 7 GENERAL INFORMATION .......................................................................................... 14

7.1 DOCUMENT HISTORY................................................................................................ 14

2 Abbreviations

CC2480 Z-Accel ZigBee Processor EM Evaluation Module IFA Inverted F Antenna ISM Industrial, Scientific, Medical PCB Printed Circuit Board

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Design Note DN0007 3 Description of the Inverted F Antenna Design

Since the impedance of the Inverted F Antenna is matched directly to 50 ohm no external matching components are needed.

3.1 Implementation of the Inverted F Antenna

It is important to make an exact copy of the antenna dimensions to obtain optimum performance. The easiest approach to implement the antenna in a PCB CAD tool is to import the antenna layout from either a gerber or DXF file. Such files are included in CC2430DB reference design [1]. The gerber file is called “Inverted_F_Antenna.spl” and the DXF file is called “Inverted_F_Antenna.dxf”. If the antenna is implemented on a PCB that is wider than the antenna it is important to avoid placing components or having a ground plane close to the end points of the antenna. If the CAD tool being used doesn’t support import of gerber or DXF files, Figure 1 and Table 1 can be used.

Figure 1. IFA Dimensions

H1 5.70 mm W2 0.46 mmH2 0.74 mm L1 25.58 mmH3 1.29 mm L2 16.40 mmH4 2.21 mm L3 2.18 mmH5 0.66 mm L4 4.80 mmH6 1.21 mm L5 1.00 mmH7 0.80 mm L6 1.00 mmH8 1.80 mm L7 3.20 mmH9 0.61 mm L8 0.45 mmW1 1.21 mm

Table 1. IFA Dimensions

Since there is no ground plane beneath the antenna, PCB thickness will have little effect on the performance. The results presented in this design note are based on an antenna implemented on a PCB with 1 mm thickness.

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Design Note DN0007 4 Results

All results presented in this chapter are based on measurements performed with CC2430DB [1].

4.1 Radiation Pattern

Figure 2 shows how to relate all the radiation patterns to the orientation of the antenna. The radiation patterns were measured with CC2430 programmed to 0 dBm output power.

XY plane

XZ plane

YZ plane

Figure 2. How to Relate the Antenna to the Radiation Patterns

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Design Note DN0007

Figure 3. XY Plane Vertical Polarization

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Figure 4. XY Plane Horizontal Polarization

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Figure 5. XZ Plane Vertical Polarization

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Figure 6. XZ Plane Horizontal Polarization

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Figure 7. YZ Plane Vertical Polarization

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Figure 8. YZ Plane Horizontal Polarization

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4.2 Reflection

Figure 9. Measured Reflection at the Feed Point of the Antenna

Figure 9 show that the IFA ensures less than 10 % reflection of the available power for a bandwidth of more than 300 MHz. A large bandwidth makes the antenna less sensitive to detuning due to plastic encapsulation or other objects in the vicinity of the antenna.

4.3 Bandwidth

Another way of measuring the bandwidth after the antenna is implemented on a PCB and connected to a transmitter is to write test software that steps a carrier across the frequency band of interest. By using the “Max hold” function on a spectrum analyzer the variation in output power across frequency can easily be measured. Figure 10 shows how the output power varies on the IFA when the PCB is horizontally oriented and the receiving antenna has horizontal polarization. This measurement was not performed in an anechoic chamber thus the graph shows only the relative variation for the given frequency band.

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Design Note DN0007

Bandwidth IFA

2.4 2.4835

-60

-50

-40

-30

-20

-10

0

2.2 2.3 2.4 2.5 2.6 2.7 2.8

Frequency [GHz]

Rel

ativ

e ou

tput

pow

er [d

B]

Output pow er

Low er band edge

Upper band edge

Figure 10. Bandwidth of IFA

5 Conclusion

The PCB antenna presented in this document performs well for all frequencies in the 2.4 GHz ISM band. Except for two narrow dips, the antenna has an omni directional radiation pattern in the plane of the PCB. These properties will ensure stable performance regardless of operating frequency and positioning of the antenna. Table 2 lists the most important properties for the inverted F antenna.

Gain in XY Plane 1.1 dB Gain in XZ Plane 3.3 dB Gain in YZ Plane 1.6 dB Reflection < -15 dB Antenna Size 25.7 x 7.5 mm

Table 2. Summery of the Properties of the IFA

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Design Note DN0007 6 References

[1] CC2430DB Reference Design (swrr034.zip)

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Design Note DN0007 7 General Information

7.1 Document History Revision Date Description/Changes SWRU120B 2008-04-04 Renamed CCZACC06 to CC2480 SWRU120A 2008-02-28 Added reference to CCZACC06 and CC2520 SWRU120 2007-04-16 Initial release.

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